Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, diamond can be deposited.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantages of limited dissociation to form active species and restriction to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for diamond film growth via a chemical vapour deposition (CVD) process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
U.S. Pat. No. 6,645,343 (Fraunhofer) discloses an example of a microwave plasma reactor configured for diamond film growth via a chemical vapour deposition process. The reactor described therein comprises a cylindrical plasma chamber with a substrate holder mounted on a base thereof. A gas inlet and a gas outlet are provided in the base of the plasma chamber for supplying and removing process gases. A microwave generator is coupled to the plasma chamber via a high-frequency coaxial line which is subdivided at its delivery end above the plasma chamber and directed around an upper peripheral region of the plasma chamber to an essentially ring-shaped microwave window in the form of a quartz ring. The invention as described in U.S. Pat. No. 6,645,343 focuses on the ring-shaped microwave window and discloses that the coupling of microwaves in the reactor chamber is distributed in a circularly symmetric fashion over the entire ring surface of the microwave window. It is taught that because the coupling is distributed over a large surface, high microwave power levels can be coupled without high electric field intensities developing at the microwave window thus reducing the danger of window discharge.
The present inventors have identified several potential problems with the coupling configuration described in U.S. Pat. No. 6,645,343 for feeding microwaves from the microwave generator into the plasma chamber:                (i) The upper housing section may be prone to over-heating in use due to high temperature plasma formed in the reactor chamber. The arrangement described in U.S. Pat. No. 6,645,343 doesn't address the issue of extracting waste energy from the upper housing section. In time this wall may become very hot leading to eventual failure of the annular window seal and of the annular window itself. The funnel shaped coaxial line entirely surrounds the upper portion of the chamber and so it is difficult to envisage how any coolant could be supplied to the upper portion of the chamber in the described arrangement.        (ii) U.S. Pat. No. 6,645,343 mentions the possibility that the connection for supplying process gas can face the substrate holder and can be directed approximately centrally towards the substrate holder. However, U.S. Pat. No. 6,645,343 does not describe any means of achieving this arrangement. As described above, the funnel shaped coaxial line entirely surrounds the upper portion of the chamber and so it is difficult to envisage how any process gases could be supplied centrally to an upper portion of the chamber and directed towards the substrate holder. The only possibility would appear to involve feeding process gases down through the central inner conductor of the coaxial feed. In the arrangement described in U.S. Pat. No. 6,645,343, the inner central conductor of the coaxial feed extends from an upper wall of the rectangular waveguide from the microwave generator to the funnel-shaped transition area. If services such as process gases and/or coolant are to be provided to the upper housing of the plasma chamber, they must be contained in a relatively restricted passage within the inner central conductor over a considerable distance.        (iii) In the arrangement described in U.S. Pat. No. 6,645,343, the inner central conductor of the coaxial feed extends from an upper wall of the rectangular waveguide from the microwave generator thus providing an electrically grounded point. Accordingly, the waveguide transition must be designed to operate with a grounded inner conductor. One of the potential disadvantages of this design is the requirement to make the distance between the grounded inner conductor and the short circuit of the rectangular waveguide to be a half guided wavelength. If not precisely configured, this can adversely affect power coupling into the chamber. Having the inner conductor of the coaxial waveguide electrically floating in the waveguide is in many respects a simpler and more convenient method of transferring power from a rectangular to a coaxial waveguide but has the disadvantage of losing the grounded point at which services such as water and gas can be introduced.        (iv) In U.S. Pat. No. 6,645,343, the ring-shaped microwave window forms a portion of the side wall of the reactor chamber between upper and lower housing sections. As such, the ring-shaped microwave window may be placed in compression by the overlying upper housing section which may cause damage to the microwave window. Furthermore, it may be difficult to easily and reliably form a vacuum seal between the upper and lower housing sections at the ring-shaped microwave window. In order to solve this problem it may be possible to modify the arrangement described in U.S. Pat. No. 6,645,343 such that the grounded inner conductor has the added function of providing a mechanical anchor point through which tension can be applied to resist the opposing pressure caused by the reduced pressure inside the cavity compared to that outside. To help achieve an effective seal it is possible to use a tensioning spring arrangement that maintains a consistent force on the annular window and its seals. However, this arrangement adds complexity. Furthermore, if it is desired to use a floating inner conductor no mechanical link is possible and an alternative must be sought.        (v) In U.S. Pat. No. 6,645,343, a complicated funnel-shaped coaxial line is described for guiding the microwaves to the ring-shaped microwave window forming a portion of the side wall of the plasma chamber. Such a complicated waveguide structure having multiple transitions is not considered to be desirable for optimum power handling and efficient coupling of microwaves into the plasma chamber.        
It is an aim of certain embodiments of the present invention to address one or more of these problems.